1. Field of the Invention
The present invention pertains to magnetic detection methods in which buoys are conveyed in a helicopter and dropped into the sea, whereupon they transmit indications from magnetometers contained in them to the helicopter, for example through a radio link. These indications make it possible to determine the presence of submerged masses, such as submarines which modify the earth's magnetic field. The invention also pertains to the buoys and to the devices placed in the helicopter to implement the method, as well as to any device that is released from a carrier vehicle and has a sensitive magnetometer.
2. Description of the Prior Art
Among the most sensitive magnetometers that can be used in devices such as this, there are known magnetic resonance magnetometers working with optically pumped helium cells. The European patent application No. 0 224 986 filed on 4th Aug. 1986 under No. 86305992.9 thus describes a device that comprises two helium.sup.3 magnetometers borne at the end of two arms fixed to one end of the device.
The helium.sup.3 magnetometers are themselves widely known, for example from the patent application No. 2 104 786 filed on 15th July 1971 as well as the European patent application No. 0 246 146 filed on 7th May 1987 under No. 87401054.9. Fundamentally, these magnetometers have a helium-filled cell which is pumped by different means from the metastable level onwards. Measuring means enable the helium nuclei or atoms to be made to precess, depending on the isotope used, around the magnetic field to be measured, and the measurement of the precession frequency enables value of this magnetic field to be determined.
It is possible to use different pumping means, but one of the most widespread of these means is optical pumping using a substantially monochromatic light source, the wavelength of which is that of the emission of helium used between its metastable level 2.sup.3s and the excited level 2.sup.3p. The oldest and simplest technique consists in the use of a discharge lamp, which is itself a helium lamp that naturally gives the right wavelength, but it is now possible to use lasers tuned to the right frequency.
The operation takes place in two stages. In a first stage, the measuring cell is excited by means of the pumping stage, and this excitation is stopped when it has reached its maximum effect. In a second stage, the nuclei or atoms of helium are made to precess, and a measurement is made of the corresponding frequency, which determines the value of the magnetic field. It can be noted, quite remarkably, that the nuclear magnetic moments oriented by the optical pumping process preserve this orientation for a relatively long period which may easily go up to ten hours and may exceed 24 hours.
The bulk and weight of the means for the excitation of the helium cell are not inconsiderable in relation to the rest of the measuring apparatus, and they consume a relatively large amount of energy, which requires a power source that is itself bulky and heavy. If it is desired to be able to convey a sufficient number of buoys in a helicopter, the size and the weight of each of these buoys has to be reduced to the maximum extent, and this often leads to limiting the size of the excitation means, and hence the excitation itself. This restricts the sensitivity and duration of the measurements.